Abstract

The Lisbon Valley fault follows a northwest strike along the crest of a salt anticline and cuts diagonally across the Big Indian Wash uranium area in San Juan County, Utah. Altered rocks collected along the fault zone have been studied microscopically and by X-ray diffraction, for significant mineralogical data with a bearing on mineralization.

Abundant kaolin is found in altered Chinle strata near the fault and extending 50 to 300 feet away. In contrast, illite, chlorite, montmorillonite, and mixed-layer illite-montmorillonite are commonly found in unaltered Chinle Shale occurring elsewhere along the Lisbon Valley anticline. Veinlets of dickite occur in the zone of shear, accompanied by tiny veinlets of pyrite and globules of indurated asphalt. Both original copper sulphides and oxidized copper minerals occur in fractured rock along the Lisbon Valley fault over a wide stratigraphic range. Rocks along the fault zone have generally been bleached, and porous Burro Canyon Sandstone has been silicified for 50 or more feet away from the fault on the hanging-wall side. Solution activity in the fault zone is indicated by bleaching, silicification, and deposition of copper sulphides. Argillic mineral associations and indurated hydrocarbons indicate that the mineralizing solutions were heated.

The western upthrown limb of the Lisbon Valley anticline contains uranium deposits which occur mostly in Triassic Chinle Shale, but also in Permian Cutler strata. The rising thermal mineralizing solutions active along the Lisbon Valley fault zone may have carried uranyl ions which were reduced and precipitated in Chinle strata. The passage of solutions to the Chinle most likely followed subsidiary fractures and certain porous beds found in the Permian Cutler Formation, known as sugar sands in the mining localities.

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